Asteroids are one of the many small or minor rocky planetoids that are a member of the solar system and that move in elliptical orbits primarily between the orbits of Mars and Jupiter. The largest representatives of asteroids are 1 Ceres, with a diameter of about 1,003 km, 2 Pallas and 4 Vesta, with diameters of about 550 km. About 200 asteroids have diameters of more than 97 km, and thousands of smaller ones exist. The total mass of all asteroids in the solar system is much less than the mass of the Moon. The larger bodies are roughly spherical, but elongated and irregular shapes are common for those with diameters of less than 160 km. Most asteroids, regardless of size, rotate on their axes every 5 to 20 hours. Certain asteroids may be binary, or have satellites of their own.

Few scientists believe that asteroids may be remains or remnants of former planet. It is more likely that asteroids occupy in a place in the solar system where a sizable planet could have formed but was prevented from doing so by the disruptive gravitational influences of the nearby giant planet Jupiter. Some asteroid orbits intersect with earth’s orbit and they are known as Apollos. Astronomers have found more than 300 asteroids with orbits that approach Earth’s orbit. Some scientists think that several thousand of these near-Earth asteroids may exist and as many as 1,500 could be large enough to cause a global catastrophe if they collided with Earth. Still, the chances of such a collision average out to only one collision about every 300,000 years.

Many scientists believe that a collision with an asteroid or a comet may have been responsible for at least one mass extinction of life on Earth over the planet’s history. A giant crater on the Yucatan Peninsula in Mexico marks the spot where a comet or asteroid struck Earth at the end of the Cretaceous Period; about 65 million years ago, which scientists think, caused the death of the dinosaurs.

The NEAR Shoemaker is the first spacecraft mission specifically designed to study an asteroid. It was launched on February 17, 1996 on a 4-year journey to the near Earth asteroid 433 Eros. Originally only named NEAR, the spacecraft was renamed after arriving at the asteroid, in honour of the late Eugene Shoemaker, a renowned planetary scientist.

The objective of the mission was to encounter and orbit Eros for one year to collect imagery and gather data on the asteroid\'s properties such as surface features, composition, and rotation. During its orbit of Eros, NEAR Shoemaker imaged the asteroid from altitudes ranging from 320 km to as low as 5 km. The mission was successfully completed in February 2001 when a previously unplanned landing was executed. This was the first time a spacecraft ever landed on an asteroid.

Asteroids can be classified into distinct types. Three-quarters of the asteroids visible from Earth, including 1 Ceres, belong to the C type, which appear to be related to a class of stony meteorites. Extremely dark in colour, probably because of their hydrocarbon content, they show evidence of having adsorbed water. There are other types of asteroids, which have not been determined yet.

Van Allen Belt.
Van Allen radiation belts are two belts of radiation outside the earth\'s atmosphere, extending from 650–65,000 km above the earth. Their existence was confirmed from information secured by launching the first U.S. earth satellite, Explorer 1, sent up during the years of 1957–58. The belts were named for James A. Van Allen, the American astrophysicist who first predicted the belts and then was first to interpret the findings of the Explorer satellite. The region of external belts has been given the name of magnetosphere to distinguish it from the atmosphere. The charged particles of which the belts are composed circulate along the earth\'s magnetic lines of force extending from the area above the equator to the North Pole, to the South Pole, and circles back to the equator. These particles are believed to originate in periodic solar flares. Carried by the solar wind from the sun, they become trapped by the earth\'s magnetic field and are responsible for the aurora borealis seen at polar Regions.

1. SciencePower 9-McGraw-Hill Ryerson limited. 1999

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